The invention herein pertains generally to mechanical bandpass filters and specifically to a mechanical filter which employs bar resonators operated in a longitudinal mode of vibration to achieve a narrower bandpass frequency range than previously attainable with prior art mechanical filters.
Although mechanical bandpass filters are well known and are extensively used commercially in electronic circuits, their application for narrow bandwidth operation has been somewhat limited because of inherent problems, such as high insertion loss and passband rounding, engendered by some of the transduced mechanical energy being diverted to the structural supports away from the output transducer stage. In addition, specific types such as conventional disk-wire designs require a high degree of resiliency (wire compliance) in order to achieve narrow bandwidths, so that as the ruggedness of the device is diminished to achieve greater selectivity a point is reached when the structure is too fragile to withstand the forces normally encountered in a commercial environment. These filters are typically capable of minimum bandwidths of about 0.1% of the center frequency. Bar-flexure designs suffer from the inaccessibility to the singular nodal point for support purposes so that some of the transduced mechanical energy must inherently be dissipated through the structural supports thereby reducing the attainable Q levels and consequently the degree of selectivity. The bandwidth for these devices generally exceeds 0.2% of the center frequency.
The longitudinal-mode of resonant response wherein a bar resonator vibrates along its longitudinal axis is a well known and understood phenomenon. However, although longitudinal-mode resonators, such as the Langevin type, comprising two columnar metal rod sections separated by a piezoelectric transducer (see Modern Filter Theory and Design by Temes and Metra, p. 175) have been employed in mechanical bandpass filters such as the tandem type described in an article entitled "The Mechanical Filter: Evaluation to Technical Maturity" which appeared in the June 1973 issued of Japan Electric Engineering, the realizable bandwidths do not even compare with those of the aforementioned filters (the article indicating a minimum bandwidth of 0.2% of center frequency).
With the foregoing in mind, it is a primary object of the present invention to provide a new and improved mechanical bandpass filter.
It is a further object of the present invention to provide such a filter which advantageously employs the longitudinal mode of vibrational response for bar resonators.
It is still a further object of the present invention to provide such a filter which exhibits a narrower bandwidth than previously attainable with prior art mechanical filters.
The foregoing objects as well as others, and the means by which they are achieved through the present invention, may be best appreciated by referring to the Detailed Description of the Invention which follows hereinafter together with the figures.